1. Field of the Invention
The present invention relates to an ultrasound imaging device, and more particularly, to an ultrasound imaging device having a transducer, with a poling polarity, to emit ultrasound waves at an object to generate an image of the object, and which prevents depoling of the transducer. Generally, the object to be imaged is an organ of the human body.
2. Description of the Related Art
Ultrasound imaging devices utilize transducers which transform electrical energy into ultrasound and visa-versa. Such transducers are commonly used for non-destructive and non-invasive testing, such as for the examination of internal organs. Transducers, and more particularly, piezoelectric transducers, are often made of ceramic or a crystalline structure. These structures are expensive to grow and even more expensive to integrate with the necessary mechanical and electrical components required for ultrasound imaging. Medical ultrasound probes cost $10,000.00 and up.
Piezoelectric transducer are polarized. That is, the fine structure of the unit cells are oriented with an electric field at high temperatures during the manufacturing process thereof. As a result, when the voltage across the transducer is changed, the various unit cells in the crystal structure strain to produce a net displacement of the material, so as to emit a pressure wave at ultrasonic frequencies. Thus, as part of the normal manufacturing process, the ceramic or crystal structure is polarized. This transducer material process is called poling and produces a material that is called poled. If the material is not polarized, the transducer generally will not work, and the transducer is not piezoelectric anymore as it will not emit ultrasound when the voltage across the same is changed because the various unit cells in the structure behave randomly and the internal strains produce no external displacement. In effect, the various groups of unit cells in the volume of the material cancel each other out when you apply the voltage. The polarization of these materials is chiefly influenced by three variables: voltage; temperature; and time.
Historically, what has been done is to transmit a square pulse which goes from 0 volts and goes negative, perhaps down to xe2x88x92170 volts. The magnitude of the voltage is adjustable, but it always goes negative, so if you pole the transducer with a negative DC source, then when an ultrasonic imaging device transmits, there is merely a repeating of what was done when the transducer was manufactured, and the poling will be maintained.
More recently, it has been found advantageous to preferentially receive and display echo signals that arise because of the non linear properties of tissues or contrast agents. It is not desirable at times to use the simple square pulse. More complicated pulses are advantageous, and in particular, the simplest replacement pulse would be a square wave consisting of a negative going pulse immediately followed by an equal and opposite positive going pulsexe2x80x94or a positive going pulse followed by an equal and opposite negative going pulse. Such a simple pulse is called bipolar. However, any pulse with both positive and negative excursions is bipolar.
One motivation to use bipolar waveforms is to reduce the amount of waveforms exhibiting second harmonic of the fundamental frequency of the desired transmitted. It is ideal to transmit with no second harmonics, and then receive the second harmonic, so that the second harmonic that is received is solely caused by an object to be imaged or the tissue non-linearities in a patient. Another non-linear strategy is to sequentially transmit a first pulse, then receive and store data about the echoes thereof, then transmit an second inverted pulse, and then receive and add data about the echoes thereof to the data previously stored. The sum is zero if the ultrasound medium and target are linear any non zero values are due to non-linearities. Since a bipolar waveform with equal plus and minus excursions, as mentioned above, has two formsxe2x80x94one way to implement the pulse inversion strategy is to alternate between them.
There are other advantages of using bipolar waveforms other than non-linear imaging. For example, with a bipolar waveform having equal plus and minus excursions there are no audible noises that come out of the ultrasound imaging transducer, so that a patient cannot hear any sounds from the ultrasound imaging device at low frequencies. Particularly, if the ultrasound imaging device is to be applied to a patient""s head, it is desirable that the patient should not hear the ultrasound wave generated to perform the imaging.
With bipolar pulses, a voltage, opposite from the voltage by which it was manufactured, is momentarily put across the ceramic. This may cause problems, such as depoling. Certainly, at a high enough voltage for a long enough time, and alternatively, also at a high enough temperature, problems occur in that the ceramic becomes depoled. This has not proven to be a significant concern with older lower-frequency transducers that use coarse elements. However, higher-frequency transducers are desired and are currently developed. As a transducer is designed to operate at higher and higher frequencies, thinner and thinner elements are required in the transducer and for a given voltage, that means that the electric field in the transducer gets higher and higher, so that the above-noted problems will only be aggravated. Additionally, different kinds of ceramic are being developed and may be extremely sensitive to depoling.
Transducer materials exhibit capacitor-like characteristics. The manufacturing operation involves applying a DC voltage to this capacitor-like structure at some specified temperature, so that each one of the unit cells in the material has a contributing dipole moment. It has a plus charge and a minus charge that are very close together, and if a field is applied and the temperature is raised, these dipoles will align with this applied external field. Then, when the transducer is cooled down and the external field is removed, the alignment remains, so that the transducer functions as a piezoelectric element. Manufacturing processes seek to align the polarity of the unit cells of the ceramic or crystal structure in a desired direction such that their strains are additive. Any external event which causes them to become randomly aligned, or less aligned would be considered a depoling event. Any external force acting on the transducer which causes it to lose its polarization causes the transducer to become less effective, and that is undesirable.
As noted previously, if the transducer is heated to a high temperature, or if a voltage opposite to the poling voltage is applied, depoling may occur. If either the heating or the application of the opposite voltage is performed for a long period of time, the depoling effect would be worse. Further, higher frequency transducer designs with thinner piezoelectrics increase the probability of depoling.
FIG. 1 shows a conventional ultrasound imaging device. A bipolar transmitter 110 generates a bipolar signal centered around 0 volts. The bipolar signal varies between +Xv and xe2x88x92Xv as shown in FIG. 2. A transmitter/receiver (T/R) switch 120 selectively connects a transducer 130 with the bipolar transmitter 110 during a transmit cycle and to a receiver 140 during a receive cycle, so that the receiver 140 is not damaged by the high voltage of the bipolar transmitter 110. During the receive cycle, any noise that is being emitted from the bipolar transmitter 110 is not seen by the receiver 140. Also, the signal to the receiver 140 is not shunted by the transmitter 110.
During the transmit cycle, the T/R switch 120 enables the transducer element 130 to receive the bipolar signal generated by the bipolar transmitter 110 so that the transducer 130 generates an ultrasound wave. During the receive cycle, the transducer 130 receives a reflected ultrasound wave back from the object being imaged and generates an image voltage signal based upon the received ultrasound wave. The T/R switch 120 enables the image voltage signal to be transmitted to the receiver 140 for processing.
The transducer 130 receives the bipolar voltage signal which goes to +Xv and then to xe2x88x92Xv, and this pattern may be repeated any number of times as a bipolar waveform. One disadvantage of this bipolar waveform is that if the transducer 130 is negatively polarized, when the bipolar voltage signal becomes positive, and is repeatedly positive over the life of the transducer 130, the material in the transducer 130 might depole. In this instance, the value of Xv may be 50 volts, for example. More versatile modalities require various pulse shapes or trains. The term bipolar pulse applies to any pulse which causes the transducer to be momentarily driven to a voltage opposite to the poling voltage. Such a voltage could depole the transducer if applied directly. Alternatively, if the transducer 130 is positively polarized, when the bipolar voltage signal becomes negative and is repeatedly negative over the life of the transducer 130, the transducer""s ceramic will depole as well. Thus, in the conventional ultrasound imaging device 100, when the bipolar voltage signal repeatedly reaches a polarity opposite to that of the transducer 130, over time, the transducer 130 may become depoled, thereby losing effectiveness.
In the past, ultrasound imaging device vendors have not been aware of or concerned themselves with the problem of the depoling of the transducers used in the ultrasound imaging devices. The present inventors have realized that the risk of depoling may be reduced if a suitable DC bias is applied to the transducer in addition to the desired pulse. The addition of this DC bias introduces no sound energy, but safeguards the poling. Thus it is possible to generate and apply any arbitrary pulse shape without danger of depoling. For example, a gaussian windowed sine wave is one such pulse shape.
An ultrasound imaging device including a transducer with a poling polarity; a bipolar transmitter which generates a bipolar voltage signal; and a bias generator which biases the bipolar voltage signal, to generate a biased bipolar voltage signal without a polarity opposite to the poling polarity of the transducer, wherein the biased bipolar voltage signal drives the transducer to generate an ultrasound wave.
A further ultrasound imaging device is described that comprising a transducer with a poling polarity; a bipolar transmitter which generates a bipolar voltage signal; and a bias generator which biases the bipolar voltage signal, to generate a biased bipolar voltage signal from having a non-zero quiescent voltage, wherein the biased bipolar voltage signal drives the transducer to generate an ultrasound wave.
A method is also presented for generating an ultrasound wave from a transducer having a poling polarity to provide ultrasound imaging, the method comprising generating a biased bipolar voltage signal which maintains a polarity the same as the poling polarity of the transducer during a transmit cycle; and supplying the biased bipolar voltage signal to the transducer, which drives the transducer to generate the ultrasound wave.